An Asynchronous Delegate Command for your WPF MVVM Apps - AsyncDelegateCommand

By Anoop Madhusudanan

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image This post assumes you are familiar with basic MVVM concepts and RelayCommand/DelegateCommand

In a lot of scenarios, you need to perform operations in an asynchronous way in your View Model. And most of the time, BackgroundWorker class comes handy. So, here is a quick implementation of an Asynchronous version of DelegateCommand, that wraps the BackgroundWorker under the hoods.

    /// <summary>
    /// An async version for delegate command
    /// </summary>
    public class AsyncDelegateCommand : ICommand 
    {

        BackgroundWorker _worker=new BackgroundWorker();
        Func<bool> _canExecute;

        /// <summary>
        /// The constructor
        /// </summary>
        /// <param name="action">The action to be executed</param>
        /// <param name="canExecute">Will be used to determine if the action can be executed</param>
        /// <param name="completed">Will be invoked when the action is completed</param>
        /// <param name="error">Will be invoked if the action throws an error</param>
        public AsyncDelegateCommand(Action action, 
                                    Func<bool> canExecute=null,
                                    Action<object> completed =null,
                                    Action<Exception> error=null
                                    )
        {

            _worker.DoWork += (s, e) =>
                {
                    CommandManager.InvalidateRequerySuggested();
                    action();
                };

            _worker.RunWorkerCompleted += (s, e) =>
                {

                    if (completed != null && e.Error == null)
                        completed(e.Result);

                    if (error != null && e.Error != null)
                        error(e.Error);

                    CommandManager.InvalidateRequerySuggested();
                };

            _canExecute = canExecute;
        }


        /// <summary>
        /// To cancel an ongoing execution
        /// </summary>
        public void Cancel()
        {
            if (_worker.IsBusy)
                _worker.CancelAsync(); 
        }

        /// <summary>
        /// Note that this will return false if the worker is already busy
        /// </summary>
        /// <param name="parameter"></param>
        /// <returns></returns>
        public bool CanExecute(object parameter)
        {
            return (_canExecute == null) ? 
                    !(_worker.IsBusy) : !(_worker.IsBusy) 
                        && _canExecute();    
        }

        /// <summary>
        /// Let us use command manager for thread safety
        /// </summary>
        public event EventHandler CanExecuteChanged
        {
            add { CommandManager.RequerySuggested += value; }
            remove { CommandManager.RequerySuggested -= value; }
        }

        /// <summary>
        /// Here we'll invoke the background worker
        /// </summary>
        /// <param name="parameter"></param>
        public void Execute(object parameter)
        {
            _worker.RunWorkerAsync(); 
        }
    }
So, now from your view model, you can use use our AsyncDelegateCommand like this.
    public class YourViewModel : INotifyPropertyChanged
    {
        public YourViewModel()
        {
            DoSomethingCommand = new AsyncDelegateCommand(
                    action:() => DoSomething(),                   
                    error:(ex)=>Debug.WriteLine(ex.Message));
        }

        public ICommand DoSomethingCommand { get; set; }

        void DoSomething()
        { 
        }
		
    }

This will execute DoSomething in an asynchronous manner. And when you bind the DoSomethingCommand to the Command property of your UI elements, you'll see that the UI element's enable/disable state is changing automatically based on our BackgroundWorker's state (i.e, when worker is busy, you'll see your buttons disabled).

Just thought about sharing the implementation after finding this pretty handy :). Not sure if some one out there already has a similar idea/implementation that wraps the Background worker inside a command implementation, if you know about any, please leave a message. A quick note – I’m using optional parameters here (C# 4.0 specific). So tailor down as required if you need to use this in 3.5.

Happy Coding!!

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